1. Field of the Invention
This invention relates to high voltage liquid-cooled apparatus and supply leads therefor and to sputtering apparatus.
2. Description of the Prior Art
Problems can arise in the operation of high voltage apparatus due to electrical breakdown in high tension leads. Such problems are particularly prevalent in cases where the high tension leads are subject, in operation of the apparatus, to conditions of high temperature and low ambient pressure.
For example, in U.S. Application Ser. No. 220,899 there is disclosed a reactive sputtering apparatus for sputtering a film of metal oxide on to the surface of a substrate, e.g. a sheet of glass. The sputtering apparatus comprises a vacuum chamber in which there are means for supporting a substrate on to whose surface a film of metallic oxide is to be sputtered. A cathode assembly is arranged in the vacuum chamber in the vicinity of the substrate, the cathode assembly comprising a series of spaced, parallel cathode strips. The surfaces of the cathode strips which face the surface of the substrate which is to be coated are formed from the metal whose oxide is to be sputtered. Each of the cathode strips has an earthed electrostatic shield which surrounds and is spaced from the cathode strip on all sides except for the sputtering surface of the cathode strip. Radiant heating means are provided in the vacuum chamber for maintaining the substrate at a desired elevated temperature during the sputtering process.
Each of the cathode strips is hollow so as to provide a water cooling of the cathode assembly. Water is fed into the first cathode strip of the cathode assembly through a flexible inlet pipe leading through the wall of the vacuum chamber. The water is passed from the first cathode strip into the next through a further flexible pipe, each of the cathode strips being connected with the preceding cathode strip in this manner. The water leaves the last cathode strip of the cathode assembly through a flexible outlet pipe which is taken out through the wall of the vacuum chamber. In an alternative arrangement, applicable to equipment of larger size, each of the cathode strips of the cathode assembly may be provided with individual inlet and outlet water pipes leading through the wall of the vacuum chamber. The water pipes are of a plastics material such as polytetrafluoroethylene (PTFE) or nylon, and are connected directly with the cathode strips by standard union couplings, usually made of brass.
Means are provided for supplying a sputtering atmosphere of oxygen and another gas or gases (e.g. argon) at reduced pressure into the vacuum chamber. Sputtering of the oxide of the cathode metal on to the surface of the substrate is obtained by applying a high negative potential to the cathode strips of the cathode assembly. Electrical connection to the cathode strips has hitherto been made by high tension leads of the co-axial cable type. A main lead may be provided connecting with the first cathode strip of the cathode assembly, the other cathode strips being connected one with another by short bridging leads. Each of the high tension leads may be connected with its respective cathode strip by a standard co-axial plug type connector. The centre conductor of the lead, carrying the high negative potential, is connected with the central socket of the connector which is a push fit with a connector pin on the cathode strip. The outer conductor of the lead is earthed and connected with the sheath of the connector, which is screwed on to the uncooled electrostatic shield of the cathode strip.
Usually the water connectors to the cathode strips give very few problems in operation compared with the connectors for the high tension leads. Problems arise with the connectors for the high tension leads due to three basic reasons:
1. Because of the need to heat the substrate to an elevated temperature (e.g. 300.degree. to 350.degree.C) in carrying out the sputtering process, everything in the vacuum chamber which is not water cooled also becomes hot. In particular, the electrostatic shield of the cathode strips and hence the connectors for the high tension leads which are coupled with the electrostatic shields through the sheaths of the connectors becomes very hot. At the high temperature involved, the insulation between the earthed shield and both the central socket of the high tension connectors and the centre conductor of the high tension lead becomes prone to electrical breakdown at the high voltages involved.
2. Because the electrical contact in the high tension connectors is by means of a push fit of the central socket of the connectors with the pins on the cathode strips, the contact is poor, and loss of resilience with temperature of the sockets in the connectors, plus dirtying by oxidation, makes contact even worse.
3. The gas pressure in the vacuum chamber is set to promote a sputtering discharge from the cathode strips. Naturally this also tends to encourage such a discharge between any other high tension point and the nearer earthed points. Such discharges occur in the high tension connectors between the central pin and socket and the earthed sheath of the connectors. This causes transfer of conducting material to the connector insulation as well as carbonising it through heating.
All these factors tend to cause breakdown of the connector insulation, first by coating and carbonisation, and then by electrical conduction across this pathway, which accelerates carbonising to cause complete failure very rapidly once initiated.
Similar problems can occur in other types of high voltage apparatus where the connectors become hot in operation.